Two common gamma-chain cytokines, IL-2 and IL-7 are currently approved or considered for both AIDS and cancer immunotherapy. See, Sportes, et al., (2008) J Exp Med 205:1701-1714; Levy, Y. (2009) J Clin Invest. 119(4):997-100785; and Rosenberg, et al., (2006) J Immunother 29:313-319. No clinical experience exists with the gamma-chain cytokine IL-15. See, Cheever, (2008) Immunological Reviews 222:357-368.
IL-15 is a non-redundant cytokine important for the development, survival, and proliferation of natural killer (NK) and CD8+ T-cells. It shares with IL-2 the same IL-2 beta gamma receptor and has many similar effects on lymphocytes, but unlike IL-2 is not produced by lymphocytes but by a plethora of other cells including, importantly, antigen presenting cells and macrophages, and stroma cells in several tissues. The biological effects of IL-2 and IL-15 at the level of the organism are dramatically different, as shown by work in knockout mice: lack of IL-15 causes immune system defects, whereas lack of IL-2 causes immune activation and severe autoimmunity. See, Waldmann, (2006) Nat Rev Immunol 6:595-601; and Ma, et al., (2006) Annu Rev Immunol 24:657-679. Both cytokines are under tight and complex regulation at all steps of expression and secretion. The biological differences of IL-2 and IL-15 are determined by their different production sites, their strength of association with membrane receptor proteins termed IL-2 Receptor alpha and IL-15 Receptor alpha (IL-15Rα), respectively, and the regulation of these extra receptor molecules. IL-15 has been also reported to have a unique mechanism of action in vivo among the common gamma chain cytokines: IL-15 functions in a complex with IL-15Rα and depends on the co-expression by the same cells of IL-15Rα. See, Burkett, et al., (2004) J Exp Med 200:825-834; Burkett, et al., (2003) Proc Natl Acad Sci USA 100:4724-4729; Dubois, et al., (2002) Immunity 17:537-547; Sandau, et al, (2004) J Immunol 173:6537-6541; Schluns, et al., (2004) Blood 103:988-994; Rubinstein, et al., (2006) Proc Natl Acad Sci USA 103:9166-9171; Bergamaschi, et al., (2008) J Biol Chem 283:4189-4199. IL-15 has non-redundant roles in the development and function of NK and intestinal intraepithelial lymphocytes (IELs). See, Cooper, et al., (2001) Blood 97:3146-3151. It stimulates cytolytic activity, cytokine secretion, proliferation and survival of NK cells. See, Fehniger, et al., (1999) J Immunol 162:4511-4520; Ross, et al., (1997) Blood 89:910-918; and Carson, et al., (1994) J Exp Med 180:1395-1403. IL-15 has a proliferative and survival effect on CD8+ memory T-cells and naive CD8+ T-cells. See, Tan, et al., (2002) J Exp Med 195:1523-1532; Zhang, et al., (1998) Immunity 8:591-599; Berard, et al., (2003) J Immunol 170:5018-5026; and Alves, et al., (2003) Blood 102:2541-2546.
Several studies have evaluated the effects of IL-15 administration in vivo. CD8+ memory T-cell proliferation increased after a single dose of IL-15 in normal mice. See, Zhang, et al., (1998) Immunity 8:591-599. Administration of IL-15 to mice enhanced the antitumor activity after syngeneic bone marrow transplantation (BMT) and antigen-specific primary CD8+ T-cell responses following vaccination with peptide-pulsed dendritic cells. See, Rubinstein, et al., (2002) J Immunol 169:4928-4935; Katsanis, et al., (1996) Transplantation 62:872-875. IL-15 also enhanced immune reconstitution after allogeneic bone marrow transplantation. See, Alpdogan, et al., (2005) Blood 105:865-873; and Evans, et al., (1997) Cell Immunol 179:66-73. The ability of IL-15 to promote growth, survival and activation of key lymphocyte populations make it also an attractive candidate for supporting growth in vitro and in vivo of cells for adoptive cell therapy. See, Rosenberg, et al., (2008) Nat Rev Cancer 8:299-308; and Berger, et al., (2008) J Clin Invest 118:294-305.
We have demonstrated that efficient production of IL-15 requires the expression of IL-15 and IL-15 Receptor alpha (IL-15Rα) in the same cell. See, Bergamaschi, et al., (2008) J Biol Chem 283:4189-4199. Co-production leads to intracellular association of IL-15 and IL-15Rα in the endoplasmic reticulum, stabilization of both molecules and efficient transport to the cell surface (FIG. 1). We showed that an additional critical step is the rapid cleavage and release of the IL-15/IL-15Rα complex from the cell surface, both in vitro and in vivo, resulting in a soluble, systemically active form of IL-15/IL-15Rα, in addition to the bioactive complex on the cell surface. See, Dubois, et al., (2002) Immunity 17:537-547; Bergamaschi, et al., (2008) J Biol Chem 283:4189-4199; and Budagian, et al., (2004) J Biol Chem 279:40368-40375. Our experiments using IL-15 complexed to a deletion mutant of IL-15Rα containing only the soluble Receptor alpha extracellular fragment demonstrated that this complex is bioactive in vivo in the absence of any membrane-bound form.
Therefore, we proposed that IL-15Rα is part of a heterodimeric IL-15 cytokine, rather than functioning as a cytokine receptor. These results have been supported by other investigators, and provide the basis for a better understanding of IL-15 biology. See, Duitman, et al., (2008) Mol Cell Biol 28:4851-4861; Mortier, et al., (2008) J Exp Med 205:1213-1225. The results also provide the molecular basis to explain some intriguing observations, including the requirement of production of IL-15 and IL-15Rα from the same cells for appropriate function in vivo. See, Sandau, et al., (2004) J Immunol 173:6537-6541; and Koka, et al., (2003) J Exp Med 197:977-984. Such results are fully explained by our finding that stabilization during co-expression in the same cell is required for physiological levels of IL-15 production. It has also been reported that the cells that physiologically express IL-15 also express IL-15Rα, consistent with IL-15 production as a heterodimer in the body. See, Dubois, et al., (2002) Immunity 17:537-547; Giri, et al., (1995) J Leukoc Biol 57:763-766; and Ruckert, et al., (2003) Eur J Immunol 33:3493-3503. Interpretation of all data available to date suggests that the main bioactive form of IL-15 is in a complex with the Receptor alpha either on the surface of the cells or in a soluble circulating form. It remains to be determined whether single-chain IL-15 is produced in the body in physiologically relevant levels and what is its exact function.
It has been previously reported that IL-15 secretion is inefficient. See, Bamford, et al., (1998) J Immunol 160:4418-4426; Gaggero, et al., (1999) Eur J Immunol 29:1265-1274; Kurys, et al., (2000) J Biol Chem 275:30653-30659; Onu, et al., (1997) J Immunol 158:255-262; and Tagaya, et al., (1997) Proc Natl Acad Sci USA 94:14444-14449. We took a systematic approach to develop IL-15 expression vectors producing high levels of bioactive cytokine based on the observation that multiple regulatory steps during gene expression create bottlenecks of IL-15 production. See, Jalah, et al., (2007) DNA Cell Biol 26:827-840; and Kutzler, et al., (2005) J Immunol 175:112-123. We showed that combination of two approaches, namely mRNA optimization (RNA/codon optimization) of the IL-15 coding sequences and substitution of the signal peptide with other efficient secretory signals resulted in synergistically improved expression and secretion of bioactive IL-15. See, Jalah, et al., (2007) DNA Cell Biol 26:827-840. Taking advantage of the stabilization of IL-15 by co-expression with IL-15Rα described above, we produced equally optimized vectors for IL-15Rα and combination vectors expressing both molecules, as well as combinations producing only the soluble heterodimeric cytokine. The final improvement in expression of secreted IL-15 was more than 1,000 fold compared to wt IL-15 cDNA, as determined by in vitro and in vivo experiments. We have produced similar vectors for mouse, macaque and human IL-15/IL-15Rα.
Two forms of interleukin-15 (IL-15) are known, containing a long signal peptide (LSP) or a short signal peptide (SSP), respectively. The two forms are produced by alternatively spliced mRNAs and differ only in the length of their signal peptides, the 48 aa long signal peptide or the 21 aa short signal peptide (120, 121, 125-127). See, Onu, et al., (1997) J Immunol 158:255-262; Tagaya, et al., (1997) Proc Natl Acad Sci USA 94:14444-14449; Meazza, et al., (1997) Eur J Immunol 27:1049-1054; Meazza, et al., (1996) Oncogene 12:2187-2192; and Nishimura, et al., (1998) J Immunol 160:936-942. Whereas LSP IL-15 is secreted, SSP IL-15 remains exclusively intracellular and its function is not known. It has been proposed that SSP IL-15 may have a regulatory function since it was detected both in the cytoplasm and the nucleus of DNA-transfected cells. The SSP signal affects both stability and localization of IL-15, since lower levels of the SSP isoform were detected when the two isoforms were expressed from similar vectors. See, See, Onu, et al., (1997) J Immunol 158:255-262; Tagaya, et al., (1997) Proc Natl Acad Sci USA 94:14444-14449; and Bergamaschi, et al., (2009) J Immunol, 5:3064-72.
In Bergamaschi, we showed that, similar to LSP IL-15, SSP IL-15 is stabilized and secreted efficiently upon coexpression of IL-15Rα in the same cell. See, Bergamaschi, et al., (2009) J Immunol, supra. Co-expression of SSP IL-15 and IL-15Rα in mice showed increased plasma levels of bioactive SSP IL-15 and mobilization and expansion of NK and T cells. Therefore, SSP IL-15 is secreted and bioactive when produced as a heterodimer with IL-15Rα in the same cell. The apparent stability of this complex both in vitro and in vivo is lower compared to LSP IL-15/IL-15Rα complex, as revealed by direct comparisons. This results in lower production of secreted bioactive IL-15/IL-15Rα. Thus, alternative splicing may provide the cell with the ability to produce different levels of bioactive IL-15. Since both forms of IL-15 may be produced in the same cell by alternative splicing, an additional level of regulation is possible. We showed that when both LSP IL-15 and SSP IL-15 are produced in the same cell they compete for the binding to IL-15Rα, resulting in lower levels of bioactive IL-15. Therefore, co-expressed SSP IL-15 acts as competitive inhibitor of LSP IL-15. This suggests that usage of alternative splicing is an additional level of control of IL-15 activity. Expression of both SSP and LSP forms of IL-15 appears to be conserved in many mammals, suggesting that SSP may be important for expressing a form of IL-15 with lower magnitude and duration of biological effects. The present invention is based, in part, on the discovery that SSP IL-15, which is produced in the thymus, is important for intrathymic effects on lymphocyte differentiation and maturation.